The present invention relates to a modified epoxy-resin-based composition which can be used as a high-performance adhesive. More specifically, the present invention relates to an adhesive composition which comprises a specially formulated epoxy resin composition and which exhibits excellent chemical and heat resistance and long-time storage stability, and thus can be advantageously used in high density IC packaging processes. The present invention also relates to improved IC packages that contain a specially formulated epoxy-resin-based adhesive composition so as to provide improved chemical and heat stability.
The trend in the consumers electronics industry is to develop products that are ever increasingly smaller, more powerful, and providing more features. As a result, tremendous pressure is imposed upon IC manufacturers to search for every possible incremental improvement that may increase the density of an IC package, while, at the same time, reducing its dimension, i.e., making the IC product thinner, shorter, lighter, and smaller, and, if possible, also reducing its cost.
Currently, two IC packing processes are most commonly employed: The tape automated bonding (TAB) method and the flexible printed circuit board (FPC) method. The former is typically used in the manufacturing of IC components for use in desktop computers, consumer electronics, liquid crystal displays (LCD), IC cards, etc. And the latter is typically used in the manufacturing of IC components for use in the automobile and communication industries, notebook computers, leadframe tapes, etc. With either the TAB or the FPC process, electronic elements are soldered onto the copper foil provided on the printed circuit board. The significant increase in the IC density causes substantial amounts of heat to be generated during operation. Such a large heat dissipation, and thus the temperature of the IC chips, will be even further increased in light of the proliferation multi-layered IC devices.
Heat dissipation typically has not been considered a serious problem since the substrates of most IC packages are made from polyimide, which is a very heat-resistant resin. However, weak spots have been noticed with respect to the adhesives that bond the conductive electronic elements to the polyimide based substrate. Most commonly, acrylic or epoxy resins are used as such adhesive. The heat resistance, or thermal stability, of acrylic or epoxy resins are not in par with polyimide. While the thermal characteristics of epoxy resin itself can be improved by modifying the composition of the epoxy resin according to several prior art teachings, such a modification of the epoxy composition has been known to result in a sacrifice in the adhesion (i.e., bonding) characteristics or other properties of the epoxy resin, and, thus, is highly undesired.
The inventors of the present invention have discovered that, by including bismaleimide into an epoxy resin based adhesive composition, the heat resistance of the resultant adhesive composition can be substantially increased. However, they also found out that the inclusion of bismaleimide created an unexpected problem in that bismaleimide exhibited long-time stability problems with elastomers such as CTBN (carboxylated acrylontrile rubber). Elastomers are added in the adhesive composition as a flexibilizer. The varnish that contained bismaleimide and an elastomer exhibited serious phase separation problems during storage.
U.S. Pat. No. 5,041,519, the content thereof is incorporated herein by reference, discloses an improved epoxy resin composition which exhibited high glass transition temperature and good toughness. The epoxy resin composition disclosed in the ""519 patent comprises a specific epoxy resin, a bismaleimide resin, and barbituric acid. While the epoxy composition of the ""519 patent provided several improved physical and electrical properties, it was designed to be used as a substrate in IC packaging; it cannot be used, and was not expected to be used, as an adhesive.
U.S. Pat. No. 5,268,432, the content thereof is incorporated herein by reference, discloses a heat resistant adhesive composition comprising an admixture of a modified bismaleimide resin, a modified polyamide-imide and a solvent. The bismaleimide resin was modified by barbituric acid and the polyamide-imide was modified by an epoxy resin. The complexity of the process disclosed in the ""432 patent underscores the difficulties and frustrations many searchers have experienced in attempting to develop an improved adhesive composition for today""s IC packing use which must be chemically and thermally stability, and at the same time, provide improved or at least same degree of adhesion characteristics. Furthermore, as discussed above, there are compatibility problems that have been observed between bismaleimide resin and elastomers. This deprives semiconductor manufacturers from taking advantage of the excellent heat resistance of using bismaleimide based resin in preparing adhesives.
The primary object of the present invention is to develop an improved adhesive composition for use in high density IC packaging processes where heat dissipation is expected to be significant. More specifically, the primary object of the present invention is to develop an improved epoxy resin based adhesive composition which can provide improved thermal and chemical stability so that they can withstand high heat dissipation expected in the manufacturing of high density and/or multi-layered IC boards, while maintaining the excellent adhesive properties that are characteristic of many epoxy resins. Because IC product is an everyday consumers item, it is particularly preferred that the improvement can be made from modifications of existing products, so that the long term environmental and health effects of the underlying chemical components are well known and well documented. With the extremely short cycle of consumers electronics, a manufacturer cannot afford the long wait for the verdict on an unaccustomed chemical, or an unfamiliar chemical combination.
The present invention is the fruit of many years of continuous and dedicated efforts by the co-inventors aiming at developing a better adhesive composition which can be comfortably and readily accepted by IC manufacturers in fabricating high density IC packages. Unexpected results were observed by the co-inventors of the present invention when bismaleimide resin is modified with barbituric acid in preparing the adhesive composition. Their results showed that, when bismaleimide was replaced with the barbituric acid modified bismaleimide, not only the excellent heat resistance of bismaleimide was retained, the resultant varnish also exhibited excellent storage stability. The improved epoxy resin based adhesive composition can be most advantageously used in bonding microelectronic components onto a polyimide substrate in the preparation of high density IC packages. As discussed above, because of compatibility problems with elastomers, adhesives containing bismaleimide resin do not show good long-term stability.
The improved epoxy resin based adhesive composition comprises the following components:
(1) A barbituric acid (BTA) modified bismaleimide (BMI) or derivatives thereof
The bismaleimide for use in the present invention is represented by one the following formulas: 
wherein R is
"Parenopenst"CH2"Parenclosest"2, "Parenopenst"CH2"Parenclosest"6, "Parenopenst"CH2"Parenclosest"8, "Parenopenst"CH2"Parenclosest"12, 
The barbituric acid (and its derivatives) that can be used to modified the bismaleimide is represented by one of the following formulas: 
wherein R1 and R2 can be independently selected from the group consisting of:
xe2x80x94H, xe2x80x94CH3, xe2x80x94C6H5, xe2x80x94CH(CH3)2, xe2x80x94CH2CH(CH3)2, xe2x80x94CH2CH2CH(CH3)2, and 
In preparing the barbituric acid-modified bismaleimide, about {fraction (1/20)} to ⅓ mole of barbituric acid per mole of bismaleimide are mixed, in the presence of an appropriate solvent (a mixture of methyl ethyl ketone and xcex3-butyrolactone) at a solid content of 30xcx9c60 wt %, and reacted at about 100xcx9c130xc2x0 C. for about 1xcx9c6 hours to complete the reaction.
(2) At least one epoxy resin selected from the group consisting of (a) tetraglycidylmethylenedianiline, (b) diglycidyl ortho-phthalate, (c) diglycidyl ether of bisphenol A, (d) polyglycidyl ether of novolac, and (e) epoxy cresol novolac.
(3) An elastomer as flexibilizer, such as carboxylated acrylontrile rubber (CTBN), polyester, acrylic, polyamide, or polyvinyl butyral.
(4) A hardening agent, which can be a mixture of methyltetrahydrophthalic anhydride (MTHPA) and ethylene glycol, DDS (diamino diphenyl sulfone), DDM (diamino diphenyl methene), BAPP (2,2xe2x80x2-biz(4-[4-aminophenoxyl]phenyl), ethylenediamine, m-phenylene diamine, trimellitic acid glycol, or polymercaptan, etc.
(5) A catalyst, which can be 1-benzyl-2-methylimidazole (1-B2Mz) or 1-cyanoethyl-2-ethyl-4-methylimidazole (2-E4Mn-CN).
(6) Methyl ethyl ketone (MEK).
(7) xcex3-butyrolactone.
(8) A surface active agent.
Other additives such as coupling agent, anti-foaming agents, UV-stabilizers, etc. can also be added to the adhesive composition of the present invention. In the final adhesive composition, it is preferred that the overall solid content is about 40xcx9c50 wt %.
The present invention discloses an improved adhesive composition for use in high density IC packaging processes where the problem heat dissipation can be expected to be significant. One of the key constraints of the present invention is that, because the end product is an everyday consumer item, the improvement must be made from existing products, so that their long term environmental and health effects are well documented and that the improved can be readily accepted by the IC manufacturing industry.
As it will be demonstrated below, an improved epoxy resin based adhesive composition has been developed based on modifications from a well-known product; it provides substantially improved thermal and chemical stability so that they can withstand high heat dissipation expected of high density and/or multi-layered IC boards, while, at the same time, it also maintains the excellent adhesive properties that are characteristic of many epoxy resins. The improved epoxy resin based adhesive composition of the present invention can be most advantageously used in bonding microelectronic components onto an polyimide substrate in the preparation of high density IC packages.
The improved epoxy resin based adhesive composition comprises the following components: (1) A barbituric acid (BTA) modified bismaleimide (BMI) or derivatives thereof; (2) at lease one epoxy resin selected from the group consisting of the tetraglycidyl methylenedianiline, diglycidyl ortho-phthalate, diglycidyl ether of bisphenol A, polyglycidyl ether of novolac, and epoxy cresol novolac; (3) carboxylated acrylontrile rubber (CTBN); (4) a hardening agent; (5) a catalyst, (6) methyl ethyl ketone (MEK) (g) xcex3-bytyrolactone; (7) a surface active agent. Other additives such as coupling agent, anti-foaming agents, UV-stabilizers, etc. can also be added to the adhesive composition of the present invention.
The process for preparing the epoxy resin based adhesive composition of the present invention is described as follows:
(1) Preparation of Barbituric Acid Modified Bismaleimide:
The bismaleimide for use in the present invention is represented by one the following formulas: 
wherein R is
"Parenopenst"CH2"Parenclosest"2, "Parenopenst"CH2"Parenclosest"6, "Parenopenst"CH2"Parenclosest"8, "Parenopenst"CH2"Parenclosest"12, 
The barbituric acid (and its derivatives) that can be used to modify the bismaleimide is represented by one of the following formulas: 
wherein R1, and R2, which can be the same or different, are respectively selected from the group consisting of:
xe2x80x94H, xe2x80x94CH3, xe2x80x94C6H5, xe2x80x94CH(CH3)2, xe2x80x94CH2CH(CH3)2, xe2x80x94CH2CH2CH(CH3)2, and 
In preparing the barbituric acid-modified bismaleimide, about {fraction (1/20)} to ⅓ mole of barbituric acid per mole of bismaleimide were mixed, in the presence of an appropriate solvent (a mixture of methyl ethyl ketone and xcex3-butyrolactone) at a solid content of 30xcx9c60 wt %, and reacted at about 100xcx9c130xc2x0 C. for about 1xcx9c6 hours to complete the reaction.
(2) Preparation of the Epoxy Adhesive Composition:
An MEK solution was prepared which had a solid content of 30xcx9c70 wt % was prepared. The solid component contains 7xcx9c25 wt % of an elastomer, carboxylated acrylontrile rubber (CTBN) containing 19xcx9c41 wt % acrylontrile, 45xcx9c70 wt % of the epoxy resin described above, a hardening agent and a catalyst. The hardening agent was provided such that it had an equivalent ratio of 0.9xcx9c1.1 relative to the epoxy resin, and the amount of catalyst added was about 3xcx9c7 phr (parts per hundred parts resin). The MEK solution was mixed with the modified bismaleimide solution prepared in step (1) to form the final epoxy resin based adhesive composition, which had a solid content of about 40xcx9c50 wt %. The hardening agent can be a mixture of methyltetrahydrophthalic anhydride (MTHPA) and ethylene glycol, DDS (diamino diphenyl sulfone), DDM (diamino diphenyl methene), BAPP (2,2xe2x80x2-biz(4-[4-aminophenoxyl]phenyl), ethylenediamine, m-phenylene diamine, trimellitic acid glycol, or polymercaptan, etc. The catalyst can be 1-benzyl-2-methalimidazole (1-B2Mz) or 1-cyanoethyl-2-ethyl-4-methylimidazole (2-E4Mn-CN). Other elastomers including polyester, acrylic, polyamide, or polyvinyl butyral, can also be used in the present invention.
Other additives such as a surface active agent, a coupling agent, an anti-foaming agent, a UV-stabilizer, etc. can also be added to the adhesive composition of the present invention. This is well known in the art and will not be elaborated.